During the New Deal administration of President Franklin D. Roosevelt, Frank served as general counsel of the Agricultural Adjudgment Administration from 1933 to 1935, and as a special counsel to the Reconstruction Finance Association in 1935. In 1937, President Roosevelt named Frank as a commissioner of the Securities and Exchange Commission. Frank served as an SEC commissioner from 1937 to 1941, including as Chairman from 1939 to 1941.
In February 1941, President Franklin D. Roosevelt named Frank as a judge of the United States Court of Appeals for the Second Circuit. He was confirmed by the Senate in March 1941. Frank was considered a highly competent judge, often taking what was perceived as the more liberal position on civil liberties issues. He served as an active judge on the court until his death in 1957.
Frank's extensive personal and judicial papers are archived at Yale University and are mostly open to researchers.
Frank died of a heart attack in New Haven in 1957.